On The Vampire Diaries Season 6 Episode 12, Caroline's former science experiment showed up with some negative results.

Colin appeared at the Forbes house to beg Caroline for help after drinking her blood took a really bad turn. Too bad she had already fed Liz her blood. After some scans at the hospital, it was clear that the vampire blood was actually making Colin's tumors grow. It was speeding up his death.

Damon concocted a plan after finding Tyler looming over Kai's body. He decided that Kai could remove the magic of the vampire blood from Liz's body since absorbing magic is what he does. Obviously Kai didn't warn Damon and Elena that the ciphon from the blood would negatively affect Liz's heart and she had a heart attack.

Magically Liz was still able to come back to life. It won't last long though and she told Stefan that he needs to watch over Caroline. 

Elsewhere, the Parker father was in town to discuss Liv and Luke's merge, which naturally the twins refused to do. Luke came up with the idea to merge with Kai, since he would do anything to save his sister. Luke lost and Kai won, then disappeared.

Elena decided life was too short to not be with Damon.